The most widely used yellow pigments are: chromium yellows, cadmium yellows and cadmiopone yellows. Since they contain lead and hexavalent chromium or cadmium, these traditional pigments are suspected of being toxic.
There are alternative pigments of the same colour based on nickel titanates TiO.sub.2 --NiO--Sb.sub.2 O.sub.3, which do not have, however, properties comparable with those of the aforementioned pigments as far as colour saturation and tinting strength are concerned.
It has also been suggested to use bismuth vanadate BiVO.sub.4 with a monoclinic crystalline structure, as an alternative pigment. As a matter of fact, said product possesses optical and pigmentary properties similar to those of "primerose" chromium yellow. However, its cost is too high considering the high price of the raw materials used for its preparation, which are vanadium and bismuth compounds.
U.S. Pat. No. 4,026,722 has suggested the use of bismuth vanadate based pigments, diluted with SiO.sub.2 and Al.sub.2 O.sub.3, in which the content in BiVO.sub.4 may be reduced down to 44% by weight. With respect to pure bismuth vanadate (and this also with respect to the traditional yellow pigments) these pigments show, however, a lowering of the colour saturation and especially a considerable drop in hiding power and tinting strength.